Direct reprogramming of differentiated human somatic cells into a pluripotent state enables the generation of patient- and disease-specific stem cells. In this study, Takahashi and colleagues generated induced pluripotent stem (iPS) cells from adult human dermal fibroblasts (HDF) and other somatic cells by retroviral transduction of four defined transcription factors: Oct3/4, Sox2, Klf4, and c-Myc. To overcome low retroviral transduction efficiency in adult human fibroblasts, the authors first introduced the mouse retrovirus receptor Slc7a1 via lentivirus before introducing the four factors with ecotropic retroviruses. Human iPS cells closely resembled human embryonic stem (hES) cells in morphology, proliferation, feeder dependency, surface markers, promoter demethylation, high telomerase activity, and global gene expression patterns. Furthermore, these human iPS cells demonstrated pluripotency by differentiating into cell types representing all three germ layers (ectoderm, mesoderm, and endoderm) in embryoid bodies, directed in vitro differentiation protocols, and teratomas in SCID mice.
Key Takeaways
Pluripotency was successfully induced in adult human dermal fibroblasts (from a 36-year-old female) using four transcription factors: OCT3/4, SOX2, KLF4, and c-MYC.
Retroviral transduction efficiency in adult human fibroblasts was increased from under 20% to approximately 60% by first expressing the mouse retrovirus receptor Slc7a1 via lentivirus.
From 5 x 10^4 transduced adult human dermal fibroblasts, approximately 10 human ES cell-like colonies were obtained around day 25 to day 30 post-transduction.
Human iPS cell lines exhibited exponential proliferation with population doubling times ranging between 43.2 and 47.8 hours, comparable to human ES cells.
Bisulfite genomic sequencing revealed that promoter regions of OCT3/4, REX1, and NANOG were unmethylated in hiPS cells, matching the active status seen in hES cells.
Transplanted human iPS cells formed teratomas in SCID mice within 9 weeks, containing differentiated tissues from all three germ layers (gut-like epithelium, cartilage, muscle, neural tissue, and epidermis).
Learning Objectives
Understand how adult human somatic cells can be directly reprogrammed into pluripotent stem cells using defined transcription factors.
Identify the four key transcription factors (OCT3/4, SOX2, KLF4, c-MYC) required for human iPS cell generation.
Analyze the cellular, epigenetic, and transcriptomic criteria that demonstrate functional equivalence between human iPS cells and human ES cells.
Evaluate experimental evidence proving pluripotency in vitro (embryoid bodies, directed differentiation) and in vivo (teratoma formation).
Glossary
iPS Cells (Induced Pluripotent Stem Cells)
Pluripotent stem cells artificially derived from adult non-pluripotent somatic cells through the expression of specific reprogramming transcription factors.
Oct3/4 (POU5F1)
A core POU-domain transcription factor essential for maintaining pluripotency and self-renewal in embryonic and induced pluripotent stem cells.
Sox2
An SRY-related HMG-box transcription factor that acts synergistically with Oct3/4 to drive the core pluripotency gene regulatory network.
Klf4 (Kruppel-like Factor 4)
A zinc-finger transcription factor that interacts with p300 histone acetyltransferase to regulate gene expression and modulate chromatin structure during reprogramming.
c-Myc
A proto-oncogenic transcription factor that functions as a booster during iPS cell induction, helping open chromatin structure for Oct3/4 and Sox2 binding.
Slc7a1 (mCAT1)
The mouse ecotropic retrovirus receptor introduced into human fibroblasts via lentivirus to enhance retroviral infection efficiency.
Teratoma
A tumor composed of tissues derived from all three embryonic germ layers (ectoderm, mesoderm, endoderm), serving as a standard in vivo test for pluripotency.
Embryoid Body (EB)
A three-dimensional cell aggregate formed by pluripotent stem cells in floating culture that mimics early embryonic lineage differentiation.
Retroviral Silencing
The transcriptional shut-off of integrated retroviral transgenes as somatic cells successfully transition into a self-sustaining pluripotent state.
Short Tandem Repeat (STR) Analysis
A DNA profiling technique evaluating specific repetitive loci to confirm cell line origin and rule out cross-contamination.
Timeline
Day 0Retroviral transduction of adult human dermal fibroblasts (HDF-Slc7a1) with OCT3/4, SOX2, KLF4, and c-MYC.
Day 6Harvesting transduced fibroblasts and replating onto feeder layers (mitomycin C-treated SNL cells).
Day 7Culture medium switched to primate ES cell medium supplemented with 4 ng/ml bFGF.
Day 14Appearance of initial granulated non-ES cell-like colonies.
Day 25Emergence of distinct, flat hES cell-like colonies.
Day 30Mechanical picking and passaging of hES cell-like colonies.
2007-11-20Publication of the research paper in Cell (Received Oct 29, Accepted Nov 12, 2007).
Mind Map
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Human iPS Cell Generation
Reprogramming Factors
Oct3/4 Sox2 Klf4 c-Myc
Transduction Strategy
Lentiviral Slc7a1 Expression
hES-like Characteristics
Pluripotency Validation
Three Germ Layer Differentiation
Direct Reprogramming of Adult Human Fibroblasts
Induction of pluripotent stem cells using four transcription factors
dna
4
Defined Reprogramming Factors
percent
60%
Transduction Efficiency with Slc7a1
grid
~10
hES-like Colonies per 5x10^4 HDF
clock
43.2-47.8 h
Population Doubling Time
activity
9 weeks
Teratoma Formation in SCID Mice
Retroviral Transgene Silencing
Retroviral transgenes were silenced in established hiPS cell lines, indicating complete reprogramming to an endogenous self-sustaining pluripotent state.
Promoter Demethylation
Bisulfite genomic sequencing confirmed CpG unmethylated status at OCT3/4, REX1, and NANOG promoters, matching human embryonic stem cells.
Multilineage Differentiation
Human iPS cells successfully differentiated into functional beating cardiomyocytes and dopaminergic neurons in vitro, and formed teratomas in SCID mice in vivo.
What are induced pluripotent stem cells (iPSCs) and why are they significant?
Induced pluripotent stem cells (iPSCs) are somatic (body) cells that have been reprogrammed back into a pluripotent embryonic stem cell-like state through the forced expression of specific transcription factors. They provide an ethically unencumbered, patient-matched stem cell platform for modeling human disease, screening drug candidates, and advancing regenerative medicine.
How closely do human iPS cells resemble human embryonic stem (hES) cells?
Human iPS cells are remarkably similar to hES cells in morphology, feeder dependency, growth kinetics, expression of surface markers (SSEA-3, SSEA-4, TRA-1-60, TRA-1-81, Nanog), promoter demethylation of Oct3/4 and Nanog, high telomerase activity, global transcriptomic profiles, and pluripotency both in vitro and in vivo.
What roles do the four transcription factors play during reprogramming?
Oct3/4 and Sox2 are essential core regulators that activate stemness genes and suppress differentiation genes. Klf4 acts in part by interacting with p300 histone acetyltransferase to modify chromatin, while c-Myc functions as a chromatin accessibility 'booster' that allows Oct3/4 and Sox2 to bind their target gene promoters in differentiated cells.
What safety concerns exist before hiPSCs can be used in clinical therapies?
Key safety concerns include genomic insertion mutations caused by integrating retroviruses (each clone carries over 20 insertion sites) and potential tumorigenesis or oncogene reactivation driven by c-Myc. Future medical applications require non-integrating delivery methods or chemical reprogramming approaches.